1. Field of the Invention
This invention relates to structural concrete members and, more particularly, to a low-cost concrete-filled reinforced-fiber composite structural member having improved strength and corrosion resistance, and to various methods for interconnecting a plurality of modular fiber-reinforced composite structural members to form framing and support structures having reduced construction and maintenance costs and resistance to seismic shock and chemical attack.
2. Description of the Related Art
Structural concrete members have found wide acceptance in a variety of civil engineering applications. The high compression strength of concrete, its low-cost and ready availability make it particularly suited for many civil applications such bridge columns, beams and support pylons. Concrete members may be prefabricated and assembled on-site using mechanical fasteners or, more typically, they may be cast in place on site using suitable form work.
For applications requiring high-strength and/or increased deformation capacity such as bridge support columns, reinforced concrete members are often used. Conventional reinforcement consists of embedded steel reinforcement bars or tensioning cables/rods running along the length of the structural member generally aligned with the member axis. Mild steel reinforcements are typically selected for use in seismic regions to maximize their inelastic deformation capacities and the ductile response characteristics of the reinforced concrete structural member in the event of seismic motion.
Pre-fabrication of such reinforced structural concrete members is possible, but due to their weight they are difficult and expensive to ship over any substantial distance. Also, heavy lifting equipment must be available on-site to position and support the structural members during assembly. On-site fabrication is also possible, but it is time-consuming and adds to the construction labor costs due to the necessity of: (1) creating a suitable temporary on-site form work to cast the concrete in the desired geometry; (2) tying the steel reinforcement, or cages (which sometimes must be welded) inside the concrete to provide adequate tensile capacity; and (3) removing and disposing of the form work once the concrete cures.
Even after the initial construction is completed, there are often significant additional costs needed to repair and/or maintain conventional steel reinforced concrete structures, particularly in areas prone to seismic activities or areas exposed to salt or other chemical agents. This is because conventional reinforced concrete, based on its design philosophy, needs to crack to transfer flexural tension forces to the steel reinforcement. These cracks form on the tension side of the concrete member as the steel reinforcement bars stretch in response to the applied load. These cracks allow water and air to enter and corrode the steel reinforcement. This corrosion of steel is accompanied by a volumetric expansion of the steel cross-section.
Over time, local corrosion of steel reinforcements around the crack area can flake-off the concrete cover and weaken the structural integrity of the concrete member, causing it to fall below required minimal standards and design capacities. Labor-intensive repair work is often required to restore the structural integrity of the member and corrosion of the steel reinforcement will typically continue even after such repairs.
Pre-stressing the reinforcement bars or providing internal support such as post-tensioning cables/rods can increase the nominal elastic strength of the reinforced concrete structural member, thereby limiting the amount of stress-induced cracking. See U.S. Pat. No. 5,305,572 to Yee. But this produces a stiffer structural member that is less able to deform and absorb energy and, therefore, more prone to brittle failure. Generally, it is desirable to retain as much ductile deformation capacity as possible, particularly in seismic areas.
U.S. Pat. No. 4,722,156 to Sato suggests the use of a pre-fabricated outer steel tube or jacket to provide a form work for concrete structural members which can be left in place as reinforcement once the concrete cures. Because the steel reinforcement tube is outside the concrete core, corrosion or other weakening of the steel reinforcement can be visually inspected and repaired.
A drawback of steel tubes, however, is that they are heavy and difficult to work with. Heavy lifting equipment is required on site to position and support the steel tubes during assembly. The added weight of steel reinforcements undesirably increases the seismic excitation mass of the structure. Skilled welders are also required to weld adjacent tube members. Such welding is undesirable because it not only adds to the overall cost of construction, but also because the welded joints are subject to brittle failure. Moreover, the resulting structure is still susceptible to corrosion damage, particularly in corrosive chemical or marine environments, since the steel reinforcement member is fully exposed. This increases the maintenance costs due to the need to periodically paint the steel tube and repair any corrosion damage.
Others have proposed replacing conventional steel reinforcement bars or tensioning rods with non-corroding composite materials such as carbon, aramid, or glass fibers maintained in a hardened polymer matrix. Such materials have shown great promise in the seismic retrofitting of existing reinforced concrete structural members such as walls, bridge columns and support pylons. See Seible, F., Priestley, M. J. N., Kingsley, G. R. and Kurkchubasche, A., "Seismic Response of Five Story Full Scale Reinforced Masonry Building," ASCE Journal Of Structural Engineering, March 1994, Vol. 120, No. 3, pp. 925-946, incorporated herein by reference. Carbon fibers are applied to the outer periphery of an earthquake-damaged concrete structural member by winding the fiber strands around the periphery of the concrete structural member while impregnating the fiber material with a suitable resin. This increases the strength of the reinforced concrete member by helping confine the concrete to prevent brittle failure. See U.S. Pat. No. 5,043,033 to Fyfe and U.S. Pat. No. 4,786,341 to Kobatake et al.
However, such composite materials have had only limited success in new construction in terms of structural effectiveness and economy. Unresolved technical difficulties such as anchorage problems and long term creep/relaxation have discouraged replacement of steel reinforcement bars with carbon fiber rods or tendons. Increased material costs several times that of conventional steel reinforced concrete members, have discouraged further research and development in this area.
On the other hand, the continuing practice of retro-fitting existing concrete structures is difficult and time-consuming. Also, the carbon fibers are generally oriented at angles nearly perpendicular to the longitudinal axis of the structural member in order to maximize the confinement strength. Thus, the fibers do not significantly contribute directly to the bending deformation capacity of the retrofitted structural member. Rather, steel reinforcement is still required. Finally, such retrofitting techniques have not addressed the issue of the connections between adjacent structural members. This is a critical consideration since the integrity of any structure composed of multiple structural members is limited by the strength and toughness of the connections which hold the individual structural members together.